Magnetic expansion valve boosts efficiency
28th July 2024
GERMANY: Siemens claims its new MVL702 magnetic expansion valve is set to redefine refrigeration cycle processes in heat pump and chiller applications.
Taking just 1sec to transition from closed to 100% open it is claimed to be the fastest valve on the market.
In addition to its speed, Siemens claims its precision and true position feedback ensure higher system stability and extra energy efficiency.
The MVL702 closes mechanically in case of power failure or cable disconnection, providing extra safety and eliminating the need for additional solenoid valves or super-cap batteries. This reduces the risk of compressor damage and results in simplified equipment design and lower equipment costs.
In addition, its A3 certification qualifies the MVL702 for use with hydrocarbon refrigerants.
A linear magnetic actuator drives the valve directly so there is no friction or mechanical play and no need for lubrication.
In tests with Refra, the Lithuanian manufacturer of chiller, heat pump and refrigeration equipment, the new magnetic expansion valve boosted the COP during heating by 8.3%.
The magnetically actuated valve achieved this result through a combination of rapid response and highly accurate positioning compared to a traditional stepper motor driven electronic expansion valve. These features allow the control system to run the circuit so that refrigerant leaving the evaporator has the lowest possible superheat.
Ultimately, the increase in COP could potentially allow a heat pump to deliver up to 21% more heat while using 14% less energy, Siemens says.
The magnetic expansion valve was tested by Refra on a double-circuit reversible air-to-water heat pump with a total heating capacity of 60kW. One of two identical circuits was equipped with the magnetic expansion valve, the other remained operating with a conventional electronic expansion valve. Both circuits were equipped with VSD compressors operating continuously at full load and providing supply water temperatures of 30 to 32°C at an outdoor air temperature of -6°C. The COP was in the average range of 3.7 to 4.1.
Each circuit was operated with an adaptive superheat controller, that continuously reduces the superheat setpoint in relation to control stability. The parameter settings were identical, being previously optimised for conventional electronic expansion valves. The tests covered the complete heating cycle including defrosting as well as the heating period.
While the performance of both circuits during the defrosting period was very similar, the circuit equipped with the magnetic expansion valve could significantly extend heating period due to higher evaporating temperature, which accounts for the doubling of the increase in COP – from 8.3% to 16.5% – across the entire cycle. To give reliable mean values from rapidly fluctuating process measurements, the cycle time was split into seven or eight identical frames for each test.
The circuit equipped with a magnetic expansion valve operated on average at 6.3K superheat, compared to 9.5K achieved by a conventional stepper motor expansion valve.
As well as boosting the COP, the lower superheat values result in higher evaporator temperatures that allow the unit to operate for longer before it needs defrosting.
